Casing treatments are known to improve stall margin on gas turbine fans and compressors. For instance, it is known to define circumferential slots in the inner surface of compressor casings adjacent the tip of a row of compressor blades. One problem associated with such casing surface treatment is that the slot bottoms or endwalls tend to burn in use. The flat endwall configuration of the slots creates flow stagnation areas which result in the formation of hot spots on the rotor casing.
Furthermore, under certain operating conditions, e.g. bird strikes, icing or hail storm, the rotor tip clearance can be much larger than the nominal tip clearance. The maximum tip clearance can be as much as four or five times of the normal running clearance. Maintaining adequate stall margin with such large tip clearances is challenging from an aerodynamic design point of view. Conventional rotor casing treatments are designed for nominal tip clearance and, thus, not adapted to effectively extend stall margin when the tip clearance is greater than the nominal value.
Accordingly, there is a need to provide an improved rotor casing treatment which addresses the above mentioned issues.